Traveling wave tube



- y 14, 1959 SUSUMU YASUDA 2,895,066

TRAVELING WAVE TUBE Filed Aug. 5, 1955 BJWOX.

Inventor S- YASUDA United States Patent 2,895,066 TRAVELING WAVE TUBE Susumu Yaslida, Tokyo, Japan, assignor to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Application August 5, 1955, Serial No. 526,651

Claims priority, application Japan October 4, 1954 -4 Claims. (Cl. 313-84) In a traveling wave tube or the similar electron tube, it hadbeen proposed that an axial focussing magnetic field be added for maintaining the electron beam emitted from an electron gun within an approximately fixed diameter throughout the length of the transit section. However, his known that this field need not be uniform throughout its length and may be satisfactory if the magnetic induction in the axial direction of the traveling wave tube or of the similar electron beam, for instance, changes periodically with a sine wave pattern.

A predominant characteristic of tube construction using such an alternating magnetic field is that the traveling wave tube device can be shortened remarkably in comparison with one using an homogenous field. Furthermore, when the field excitation is made by permanent magnets the weight can be reduced about one thirtieth in comparison with the case where focussing electro-magnetic coil which would produce an homogenous field is used.

However, on the other hand, the relation between the maxima magnetic induction regions, B max. along the axis of the alternating field, and the electron beam penetration factor p of a tube, for instance the variation of P towards the various B maxima as shown in Fig. 1, is quite critical, resulting in manufacturing difiiculties.

The present invention provides a simple additional device to adjust mechanically such critical characteristics, and to thus avoid the difficulties resulting from the close mechanical tolerance otherwise required. in traveling wave tubes voltages of several thousand volts may be impressed on the helix electrode and a slight variation of penetration factor will cause burning out of the helical electrode making the tube life very short.

The above mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, in which:

Fig. 1 shows a relation between magnetic induction B max. and an electron beam penetration factor of a tube, in the case where an alternating magnetic field is used for electron focussing and;

Fig. 2 represents a partial sectional view used to explain a practical example of the invention.

Turning first to Fig. 2, there is shown at 1 a portion of a traveling wave tube envelope and a diagram of the helical electrode mounted therein. The cathode electrode, collector electrode and associated input and output couplers are not shown since they are not necessary to an illustration of this invention. Coaxially of tube 1 are provided permanent magnets 2 of annular form all positioned so as to have their poles aligned in alternately opposite directions along the length of the envelope. Magnets 2 are all of the same length and interposed between each of the magnets are provided annular pole piece members 3 having external diameters coinciding substantially with the external diameters of the magnets 2 and inner diameters fitting closely to the envelope of ice tube 1. This construction provides an approximate sine wave distribution of magnetic field axially along the length of tube 1. A sleeve 4 of non-magnetic material is mounted outside the magnet and yoke assembly 2, 3, and of such dimensions that it may be slid axially along the length of the assembly. In this outer sleeve are provided grooves 5 which may be threaded and engaging the threaded portions are rings 6 of magnetic material of high permeability, such as iron, and designed so that they may be adjusted lengthwise of the grooves by the screw thread adjustment.

With this construction it can be seen that if the maximum field strength portions are too strong in relation to the desired penetration factor, the value of the maxima along the line may be adjusted to correct for this factor. Thus the maxima, as illustrated by a and b on the central graph of Fig. 1, can be adjusted to its most desirable position by movement of the sleeve 4 and the threaded shunting element 6 longitudinally of the magnetic structure. It will be recognised that rings 6 produce a partial bypassing of the magnetic field external of the tube structure in accordance with the longitudinal position of these rings with respect to the ends or poles of the associated magnets 2. Thus the proper distribution of the magnetic field with the tube may be achieved even if the mechanical assembly has not been precisely made.

It will be realised that when the sleeve 4 is positioned so that rings 6 are immediately above pole piece yoke 3 the by-passing of magnetic flux external of the system will be at a minimum, and when the rings are precisely centered between the yokes 3 the maximum by-pass eifect is achieved. Since all of the rings 6 are threadedly engaged on the external surface of sleeve 4, adjustment of the sleeve will assure that similar adjustments are made between each of the magnetic pole pieces. Thus, if each of the magnets and yokes is precisely the same as the other then complete uniformity will be preserved regardless of the adjustment of sleeve 4. In such a case ring 6 could be permanently fastened in place on sleeve 4 instead of being mounted for threaded adjustment.

In actual construction however it is very diificult to achieve complete uniformity of all of the elements to be assembled. Accordingly, as an initial adjustment in order to achieve the proper uniform field distribution elements 6 may be made simply by turning these elements about the threaded portions of sleeve 5 so as to obtain the proper uniform field variations. Once this is achieved the control of the maximum fields within the envelope can be obtained simply by sliding sleeve member 4 longitudinally of the assembly.

In the manufacture of this equipment it is preferable that once members 6 have been properly adjusted they be fixed in place so that they will not he accidentally displaced in use. In fact the proper centering and adjustment may be made by other means than the screw threaded arrangement shown if the magnet assembly is to be permanently associated with a particular tube structure.

While the explanation given above has been made only in connection with traveling wave tubes, it will be clear that it may be applied to other types of structure wherever it is desired to have a magnetic focussing field coaxial with an electron beam.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

l. A structure for focussing an electron beam over a relatively long path comprising an assembly of a plurality of permanent magnets positioned end-to-end along said path and with the poles aligned in alternately opposite directions and magnetic permeable elements between the ends of adjacent magnets and surrounding said path whereby a magnetic field is produced longitudinally of said beam path having effective undulations of concentration along said path, characterized by means for controlling said undulations comprising, magnetically permeable members for each magnet external of the assembly of said magnets and permeable elements, means for mounting said members closely adjacent to respective magnets and intermediate the ends thereof and means for simultaneously moving said members along the length of each magnet.

2. A structure according to claim 1, wherein said permanent magnets are annular in shape and positioned to surround said beam path, and said elements each are annular with an outer periphery substantially the same as the outer periphery of said magnets and with an inner periphery substantially smaller than the inner periphery of said magnets, characterized in that said members are ring shaped and coaxial of said magnet and said means for mounting said members comprises a sleeve of non-magnetic material slidable on the outer periphery of said permanent magnets.

3. A structure according to claim 2, further characterized in that said rings are provided with means for individual longitudinal adjustment of said rings relative to said sleeve.

4. A structure according to claim 3, further characterized in that said means-for-individual adjustment comprises threads on the outer periphery of said sleeve and engaging threads on said rings.

References Cited in the file of this patent UNITED STATES PATENTS 2,200,039 Nicoll May 7, 1940 2,212,206 Holst et a1 Aug. 20, 1940 2,431,077 Poch Nov. 18, 1947 2,442,975 Grundman June 8, 1948 2,741,718 Wang Apr. 10, 1956 2,790,920 Todd Apr. 30, 1957 2,797,360 Rogers et a1 June 25, 1957 2,812,470 Cook et a1 Nov. 5, 1957 FOREIGN PATENTS 1,080,230 France May 26, 1954 

